netfilter: ipset: remove rcu_read_lock_bh pair from ip_set_test

Callers already hold rcu_read_lock.

Prior to RCU conversion this used to be a read_lock_bh(), but now the
bh-disable isn't needed anymore.

Cc: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>